1. Field of the Invention
The present invention relates to a fixing device, and an image forming method and an image forming apparatus using the fixing device.
2. Description of the Related Art
An image forming apparatus, such as a printer, a facsimile, or a copier, forms an image including a character or a symbol on a recording medium, such as paper, cloth, or an OHP (overhead projector) sheet, based on image information. An electrophotographic image forming apparatus capable of forming a high-resolution image on a plain paper sheet at high speed is particularly widely used in offices. Such an electrophotographic image forming apparatus typically uses a heat fixing process in which toner on a recording medium is fixed thereto by heating and melting the toner and applying a pressure to the melted toner. The heat fixing process is used to provide high-speed fixing and high-quality fixed images. However, toner heating in the heat fixing process consumes approximately half or more of total power consumed by the electrophotographic image forming apparatus.
In view of current environmental issues, a fixing device with low power consumption (an energy-saving fixing device) is desirable. Specifically, it is desirable to significantly lower the heating temperature to fix toner or eliminate the need for toner heating altogether. A non-heat fixing method in which toner is fixed onto a recording medium without heating the toner at all is ideal in having low power consumption.
One example of the non-heat fixing method is a wet fixing method. The wet fixing method uses an oil-in-water fixing agent in which an organic compound that dissolves or swells toner and is insoluble or hardly soluble in water is dispersed in and mixed with water. The wet fixing method involves spraying or dropping of the oil-in-water fixing agent onto unfixed toner located at a predetermined position on the surface of an object, thereby dissolving or swelling the toner, and drying of the object.
However, the wet fixing method using such an oil-in-water fixing agent causes the following problem: When a large amount of the fixing agent is applied to unfixed toner, a recording medium (object) such as a transfer sheet absorbs water contained in the fixing agent, thereby causing creases and curls on the recording medium and significantly disturbing stable and high-speed conveyance of the recording medium, which is desirable for the image forming apparatus. When a drying device is used to evaporate and remove a large amount of water from the fixing agent applied to the recording medium, power consumption becomes comparable to that of the image forming apparatus using the heat fixing process.
As a fixing agent that does not repel water-repellent treated unfixed toner, there are lipid fixing agents prepared by dissolving a material that dissolves or swells toner in a lipid solvent. For example, one fixing agent is prepared by diluting (dissolving), for example, an aliphatic dibasic acid ester including a component that dissolves or swells a resin component, which is a constituent of toner, in a diluent (solvent) of nonvolatile dimethylsilicone.
Another example fixing agent is a fixing solution for use in a fixing method of easily and sharply fixing an electrostatically formed unfixed image onto an image reception sheet without causing image irregularity. The fixing solution is prepared by mixing 100 parts by volume of a solvent dissolving toner compatible with 8 to 120 parts by volume of a silicone oil.
Such lipid fixing agents include a lipid solvent having a high affinity with water-repellent treated unfixed toner, and therefore dissolve or swell and fix the water-repellent treated unfixed toner onto a recording medium without repelling the toner.
Each of the above-described fixing actions is performed by applying the fixing agent to an unfixed toner layer. Specifically, as illustrated in FIGS. 1A and 1B, a fixing agent is applied to an unfixed toner layer 3 on a recording medium 2 using an application roller 1 serving as a contact application unit. A fixing agent layer 4 is provided on the application roller 1. Arrow X shown in FIG. 1B indicates a direction of internal flow in the fixing agent layer 4. However, in this configuration, when the thickness of the fixing agent layer 4 on the application roller 1 is reduced to less than that of the unfixed toner layer 3 to apply a small amount of the fixing agent to the recording medium 2, unfixed toner particulates are attracted by the surface tension, which is exerted in a direction indicated by arrow Y shown in FIG. 1B, of the fixing agent layer 4 on the application roller 1 at a position where the application roller 1 separates from the recording medium 2, thereby causing offset of the toner particulates to the application roller 1 and image irregularity on the recording medium 2.
By contrast, as illustrated in FIG. 2, when the thickness of the fixing agent layer 4 on the application roller 1 is sufficiently greater than that of the unfixed toner layer 3, the surface tension of the fixing agent layer 4 on the application roller 1 directly exerted on the toner particulates becomes small at the position where the application roller 1 separates from the recording medium 2 due to the large amount of the fixing agent. Therefore, toner offset to the application roller 1 is prevented. However, since the large amount of the fixing agent is applied to the recording medium, the toner particulates flow onto the recording medium 2 due to the excessive fixing agent, thereby degrading image quality, prolonging drying time, and causing a problem in terms of fixing response.
In addition, the recording medium such as a paper sheet acquires a significant residual liquid feeling (a wet feeling obtained when a hand contacts the paper sheet). Also, when a large amount of the fixing agent containing water is applied to a cellulose-containing medium such as a paper sheet, the medium may curl up, thereby possibly jamming as it is conveyed through the image forming apparatus.
Therefore, in such application of the fixing agent using a roller, it is extremely difficult to achieve both light application (application of a small amount) of the fixing agent to a toner layer on a paper sheet for the purposes of improving fixing response, reducing a residual liquid feeling, and preventing curling of the paper sheet, and prevention of toner offset to the roller. Even with the contact application unit such as a die coating unit, a blade application unit, or a wire bar application unit, toner is offset to the contact application unit due to the surface tension of the fixing agent as the fixing agent is reduced to a small amount, thereby degrading the image quality.
As described above, with typical application of the fixing agent using the contact application unit, it is extremely difficult to achieve both light application of the fixing agent to the toner layer on a paper sheet for the purpose of improving fixing response, and uniform application of the fixing agent without causing image irregularity. Such a problem occurs in any configuration involving applying a liquid fixing agent to a layer of resin-containing particulates on a medium, as well as the toner on a recording medium.